Photographic images can be produced by imagewise exposing a photographic element comprising light sensitive emulsion layers capable of producing a developable latent image, said emulsions including silver halide grains dispersed therein. Light sensitive silver halide grains have, in general, a significant intrinsic sensitivity only to ultraviolet, violet and blue portion of the electromagnetic spectrum while the human eye is sensitive to the portion which lies approximately between 400 and 750 nm (the visible spectrum). To extend photographic sensitivity to wavelenghts longer than those naturally absorbed by silver halides, that is to the green and/or the red portions of the visible spectrum, and thereby to better approximate the image seen by the human eye, it is normal practice in conventional photography to add optical (or spectral) sensitizing dyes to the silver halide emulsions. These dyes are adsorbed on the surface of silver halide grains and render them sensitive to light absorbed by the dye. In other than conventional photography, optical sensitization is also used to extend the sensitivity of the silver halide grains to other than the visible spectrum such as infrared.
Optical (or spectral) sensitization is distinguished in the art from chemical sensitization which consists of adding silver halide grains with chemical substances which are capable of forming intrinsic sensitivity centers. Optical sensitization consists of having the surface of a silver halide grain adsorbed with a sufficient quantity (well-known in the art) of optical sensitizer dye which absorbs the light of certain wavelengths and transfers the absorbed energy to the same silver halide grain (or crystal) which adsorbed the dye. It is believed that the energy absorbed by the dye is transmitted by the dye to the same dye-adsorbing silver halide grain in the form of electrons or different energy which can be captured by the sensitivity centers of the grain to form specks of latent image. It is believed that grains of the highest sensitivity from modern emulsions need to absorb at least four photons to be rendered developable. In turn this indicates that the minimum size of a developable latent image speck is about four silver atoms (see G.C. Farnell and J.B. Chanter, The Quantum Sensitivity of Photographic Emulsion Grains, Journal of Photographic Science, Vol. 9, 73-83, 1961).
From the above, it is clear why the speed or (sensitivity) of the silver halide emulsions is normally directly related to the size of their silver halide crystals: the larger the size, the higher the probability of absorbing the at least four photons which are necessary for a single crystal to become developable (also in low exposure conditions) upon formation of a developable latent image speck.
As a matter of fact,recent developments in high speed photography make use of coarse optically sensitized silver halide crystals, including thick and thin tabular crystals, which have the characteristic of being capable of offering the maximum capture surface per weight unit of silver. Particularly, in the case of tabular crystals, said crystals need to have adsorbed great quantities of sensitizer dye to be capable of absorbing the photons which impinge on their large (but transparent) capture surface.
Fine silver halide emulsions, such as those normally known in the art as Lippmann emulsions, have not played a significant role in high sensitivity photography, although they can be optically sensitized, as known in the art. These fine grain silver halide emulsions have been recently used in association with coarse silver halide grains to absorb undesired chemical compounds or moieties which are released during the development of such coarse grains. As a matter of fact, the dimensions of the grains of such fine grain emulsions are such that they would not form an image upon exposure and development as used in conventional photography such as, for example, x-ray and color photography. In this sense, we may refer to such fine silver halide emulsions and grains as light insensitive silver halide emulsions and grains as opposed to the light sensitive silver halide emulsions and grains (including tabular grains) which are used in conventional photography to obtain x-ray and color images (the above "light insensitive" and "light sensitive" language will be used hereinafter and further specified).
The process for forming a photographic image in modern low silver photography makes use of optically sensitized light sensitive silver halide grains, preferably thin (or high aspect ratio) tabular grains. These grains are exposed and then developed to form images upon reduction of silver ions to metallic silver. If the reduction is made with a hydroquinone developer, a black and white image is formed and, if the reduction is made with a p-phenylene diamine developer in the presence of a color former (coupler), a colored image is formed.
As known in the art, the silver halide grain, to be optically sensitized (that is, made capable of absorbing exposing light), has to adsorb on its surface the sensitizer dye. Since, however, silver halide emulsions are added with further substances, --such as antifoggants, stabilizers, development accelerators, toner agents, antihalation and acutance dyes, which are adsorbed on the surface of the silver halide grain--, optical sensitization may be negatively affected by displacement of the sensitizer dye by such substances. On the other hand, the presence of sensitizer dye molecules adsorbed on the surface of one silver halide crystal may negatively affect the photographic properties (other than optical sensitivity) of the silver halide crystal by negatively affecting proper adsorption of proper agents on the same surface of the considered crystal.